The variations in isocenter position with table and gantry rotation were found to be < 0.4 mm with a compounded accuracy of â¤1.0 mm. the deviations of the ...
Measurement of Physical Aspects for Linear AcceleratorBased Stereotaxy E. M. Attalla†, A.M.Sallam ‡, I.H.Ibrahim ‡, R. A. Elawady† † Radiotherapy and Nuclear Medicine Department, National Cancer Institute, Cairo University, Egypt. ‡ Physics department, faculty of science, Ain Shams University, Egypt. ABSTRACT
T
HE purpose of this paper is to present the dosimetry and mechanical accuracy of the
dedicated Siemens PRIMUS M6/6ST linear accelerator-based Stereotactic radiosurgery and radiotherapy (SRS/SRT). The Primus M6/6ST has a single 6-MV beam with the same beam characteristics as that of the mother unit, the Siemens. The dosimteric data were taken using pin point ion chamber. The cone sizes vary from 12.5 to 40.0 mm diameter. The mechanical stability of the entire system was verified. The variations in isocenter position with table and gantry rotation were found to be < 0.4 mm with a compounded accuracy of ≤1.0 mm. the deviations of the error of readjust LTLF according to RLPP was found 0.94, 0.47, and 0.26 mm for Vertical, AP, and Lateral position respectively. The accuracy of Patients' immobilization was verified .The beam profiles of all cones in the x and y directions were within ± 0.5 mm. the physical size of the cone were within ±0.2 mm. The penumbra widths are typically 2.38 mm. The basic dosimetry parameters such as tissue maximum ratio (TMR), off-axis ratio (OAR) and cone factor needed for patient treatment were evaluated. The discrepancy between measured and calculated TMR was limited to less than 0.05 %. off-axis ratio (OAR) The film and ion chamber provided nearly identical data. The mechanical and dosimteric characteristics including dose linearity of this unit are presented and found to be suitable for SRS/SRT. The difficulty in absolute dose measurement for small cone is discussed. Keywords: SRS/SRT / cones / dosimetry/ mechanical stability
INTRODUCTION Stereotactic irradiation is the term used to describe focal irradiation techniques that deliver a prescribed dose of ionizing radiation to preselected and stereotactically localized lesions. The combined use of Stereotaxy and irradiation in treatment of disease was introduced in the early 1950`s by the Swedish neurosurgeon Lars Leksell who also coined the term radiosurgery to describe the technique (1). Several new radiosurgery approaches have been introduced in the 1980`s, all having isocentric linear accelerator (linacs) as the source of radiation. Isocentric linacs were first proposed as viable radiation sources for radiosurgery by Larsson et al. in 1974 (2). The earliest report on clinical linac-based radiosurgery was published almost a decade later by Betti and Derechinsky who developed the multiple non-coplanar converging arcs radiosurgical technique (3). Soon thereafter the non-coplanar converging arcs technique was introduced clinically in Vicenza by Colombo et al. (4) and in Heidelberg by Hartmann et al. (5) In 1986, Harvard University in Boston and McGill University in Montreal were the first two institutions in North America to use noncoplanar linac-based radiosurgery. Harvard adopted the multiple noncoplanar converging arcs technique (6) while McGill developed its own radiosurgery technique, referred to as dynamic stereotactic radiosurgery
(7)
. In the early 1990`s other, more sophisticated, approaches to linacbased radiosurgery were proposed using miniature high energy linacs as the source of radiation. The miniature linacs produce 6 MV x-ray, operate in X-band frequency region (~10,000 MHz) and, in contrast to the standard isocentrically-mounted S-band linacs (2856 MHz), are installed either on a robotic arm (8) or in a CT gantry (tomotherapy) (9). The linear accelerator-based SRS has employed multiple arc therapy to achieve uniform high dose to the target and rapid dose fall-off in normal tissues (10). In non-dynamic treatment approaches, the couch is positioned at a given angle, and arc treatment is delivered by rotating the gantry through specified start-point/stop-point combinations. Sequential changes are then made in the arc treatment by changing the couch and gantry angles. A high degree of precision is desired in a radiosurgery as well as fractionated radiosurgery (11). Various stringent QA protocols ensure that isocenter position and target center are matched at the submillimeter level. The use of conventional linear accelerators requires additional QA and significant allocation of time to ensure the positional accuracy for treatment. Such processes necessitate significant downtime before each SRS/SRT use for installation, alignment, and QA. SRS/SRT system becomes extremely desirable for fractionated treatment as it eliminates the trouble of retrofitting collimating systems, scheduling, intensive quality assurance (QA), and other comprehensive physics chores needed for a precision therapy. It eliminates the possible errors, saves time and efforts that are needed for clinical use. The pre-treatment machine preparation time could be minimized significantly with a maximum through-put, since there is no need for assembling, aligning and verifying the various precision components. The external beam stereotactic irradiation is divided into two categories: stereotactic radiosurgery SRS, in which the total dose is delivered in a single session, and stereotactic radiotherapy SRT, which, like standard radiotherapy, delivers the total dose in multiple fractions (12). Stereotactic techniques are more applied to intracranial structures since the cranium lends itself to a relatively simple and stable fixation of stereotactic frames and, moreover, the accuracy required for probing intracranial structures is generally greater than that required for other parts of the body. The Primus M6/6ST provides the necessary features of a required SRS/SRT system. The mechanical and beam characteristics of this unit along with sample beam data for SRS/SRT are presented(13). The commissioning of isocentric linac-based radiosurgery covers three areas; two related to radiation characteristics of linac and one covering the mechanical characteristics of linac. Two sets of beam measurements are
required when introducing linac-based radiosurgery in a clinical department: (1) the measurement of basic physical parameters of stationary beams collimated for radiosurgery and (2) the measurement of the dose distribution obtained with the radiosurgical technique to be used clinically. The first measurement set provides the basic data for the 3D treatment planning software, and the second set serves in test of treatment planning software and verifies the important the mechanical characteristics of linac, since the measured accuracy of the dose delivery technique depends on the alignment of the radiosurgical collimator as well as on the mechanical condition of the gantry and couch rotation. In addition to the absolute calibration of stationary radiosurgical beams, there are several relative parameters of these beams that must be measured for use in radiosurgical treatment planning systems. These parameters are essentially the same as those used in the characterization of standard large field radiotherapy beam: tissue maximum ratio TMR, relative dose factors (output factors) and off axis ratio OAR (beam profiles). These parameters are measured in tissue equivalent phantoms such as water or polystyrene, for each individual radiosurgical collimator. The measurement techniques are similar to those used in standard radiotherapy except that spatial resolution requirements for radiation detectors are much more stringent for measurement in radiosurgery because of small size of radiosurgical beams. Thus, detectors which are perfectly acceptable for measurement in standard radiotherapy may not necessarily be adequate for measurement in radiosurgery. (12 MATERIALS AND METHODS The LINAC System Stereotactic radiosurgery and radiotherapy (SRS/SRT) Siemens (PRIMUS M6/6ST) linear accelerator was installed in Radiotherapy and Nuclear Medicine Department of National Cancer Institute and used for this study. The Primus M6/6ST unit has 6MV x-ray beam. The SRS/SRT collimator is fixed to the face plate of the collimator at the time of commissioning the unit. The tertiary collimator consists of the actual cones (10 cm long). The cones are tapered for divergence. The beam characteristics of such a unit were comparable to the SRS unit. The Primus M6/6ST utilizes a small field flattening filter to provide uniform field at various depths. The weight of the accelerator head is significantly reduced by the elimination of the jaws and associated accessories which improve the rotational precision of the unit. This unit is specially designed for dose and dose rate linearity with respect to monitor unit (MU) and gantry rotations. As only small fields are needed for radiosurgery, the regular flattening filter of an accelerator is not required. The SRS System
The hardware for stereotactic radiosurgery unit from RADIONICS © (Burlington, MA) stereotactic radiosurgery system has been installed in our department on Siemens 6 MV (Primus M6/6ST) linear accelerator. The system consists of 12 circular shaped cones ranging from 12.5 to 40 mm diameter in 2.5 mm steps. The cones are inserted in a base plate mounted on the collimator head and are used for arc treatments. The Radionics threedimensional treatment planning system (RSA, Radionics, Inc., Burlington, MA, USA) that employs the Xknife RT2 dose algorithm is used for treatment planning. A siemens Somatom Plus CT scanner using magnetic optical disk (MOD) and Philips advantage MRI systems through Dicom connection are used for imaging, target volume delineation, and volume reconstruction. Each system is individually tested for accuracy and reproducibility. To test the system for geometrical accuracies, four ultra-fine green lasers (0.2mm) mounted on the walls and ceiling are utilized. The accuracy of gantry, and couch rotation was accurately verified during commissioning of the SRS facility. The couch-mounted head assembly, known as linear accelerator couch mount adapter (LCMA) is permanently installed and verified for position. The accuracy in alignment and verification of isocenter and target was carried out as described in Radionics operator manual, Tsai et al. (14) and AAPM Report No.54 (15). Routine quality assurance includes the visual and film test verification of the isocenter position with the Radionics, mechanical isocenter standard (MIS), laser target localizer frame (LTLF) rectilinear phantom pointer (RLPP) and the radiopaque ball pointer. To check the gantry rotation, the MIS is mounted on the floor bearing (couch rotational axis) of the linear accelerator. The lasers are adjusted to the MIS with the use of the scribed lines on the laser target pointer (LTP). The LTP is rotated to the right, left and ceiling and all three pairs of lasers are adjusted to the scribed lines of the LTP. The laser target pointer is replaced with a radiopaque ball pointer and aligned in the center of a small (12.5 mm) cone as shown in (figure 1and 2). Films are taken with various gantry angles (0, 90 and 270º). The optical comparator is used to check the radial centers of ball and cone. The isocenter of the couch rotation is verified with the RLPP and LTLF mounted on the LCMA. The lasers are checked to be within scribe lines of the laser target pointer. The position of the couch is adjusted to the lasers. The three microadjustments of LCMA provide small changes needed in anterior-posterior, laterals, and vertical directions. Film test is performed with the radiopaque ball in various couch angles (0, 90 and 270º). Again the films are analyzed with the optical comparator. the laser target localizer frame (LTLF) rectilinear phantom pointer (RLPP) are used for determined the error in Target Setup. Depth Helmet (DH) is also used in SRT to ensure that the relocatable frame is accurately placed in each daily fractions.
Dosimetry The three dosimteric parameters such as tissue maximum ratio (TMR), off axis ratio (OAR) and cone factor (OF) which the only factors needed in SRS/SRT dosimetry were accurately measured. The TMR is the ratio of the absorbed dose at depth d and the absorbed dose at dmax for the same source detector distance SDD. TMR is measured with a pin-point ionization chamber by using Wellhofer dosimetry system as a percentage depth dose PDD and computed by using the dosimteric software system, as long as it is relatively small compared to the field size at the depth of measurement. The OAR is the ratio of dose on a point at depth d, to the dose for the same field size and depth at the central axis. The OAR is measured as the beam profiles, which have a high dose gradient for SRS/SRT cone. The beam profiles were measured with high resolution (smaller step), minimum detector size i.e. pinpoint ionization chamber and film, both of them measured at depth (5cm). Similarly, the absolute dose delivered by cone should be measured accurately. The calibration correction factor (M) is the absolute output (cGy/MU) for the calibration field size in the isocentric geometry (SAD) at depth dmax. The machine is calibrated to deliver 1 MU/cGy at depth dmax with 100cm SSD, and then the following equation should describe M in the isocentric (SAD) geometry: In the isocentric (SAD) geometry:
SSD d max cGY M SAD MU 2
(1)
For the system already calibrated at SAD, M = 1.0, M can also be measured by delivering a known number of MU, using the calibration field size with the isocentric geometry;
M
D(d max , sref ,0) MU
(2)
The relative output factor is required for the measurement of the cone factor, where the cone attached to 7 cm x 7cm collimator field. Relative means that we compare the dose measured for one collimator s to the dose measured in the reference calibration condition (usually10cmx10 cm)Which characterizes the scatter due to both Jaw scatter and phantom scatter and is measured in water phantom with the detector at isocenter at dmax where equivalent square s determined by jaws only.
D (d max , s,0) S t s D ( d , s , 0 ) max c
(3)
where Scal Side of the equivalent square at the calibration jaw setting (without collimator assembly).The Jaw scatter factor (Sj )which characterizes the effect of fluence variation due to scattered radiation from the collimator and is measured by measuring total scatter and then dividing by phantom scatter for each collimator during the fitting process, and the phantom scatter factor Sp(s) which characterizes the scatter in water phantom defined as the ratio of the dose rate for a given field at reference depth d max(the housing collimator attached to the gantry and set the Jaw at square field size7x7 cm ) to dose rate at the same depth for the reference field size(housing collimator removed from the gantry and set the Jaw at square field size7X7cm) . Sj =
Sp=
D(d max , s jaw , snet ,0,0,0)
(4)
D(d max , sopen , snet ,0,0,0) D (0,0, d D (0,0, d
max max
,s
,s
open open
,s
,s
net
)
open
)
(5)
Where Sjaw is the side of the equivalent square of the jaw field , Snet is the side of the equivalent square for the radiation field which reaches the patient (jaws +collimator),and Sopen is the side of the equivalent square at the Jaw Open setting (without the collimator assembly). In the jaws dose algorithm, the effect of the jaw penumbra is treated independently from the circular collimator penumbra effect contained within the OAR data. Additionally, the scatter factor utilized in the jaws dose algorithm is divided into two components(1) The LINAC head scatter due to the jaws (SJ),(2)The Phantom Scatter ( SP) Both of these are normalized to the Jaw Open geometry (e.g. 7 x 7). SJ represents the change influence output of the machine, which is mostly affected by the jaw setting. As the jaws are closed, more radiation is scattered back to the LINAC's monitor chambers, causing the dose delivered to the patient to be attenuated. SP represents the change in output that results from the change in the size of the radiation field hitting the patient. This is a function of the field defined by the collimator and the jaws. Xknife uses different dose algorithms for beams with jaws and without jaws. [I] Non-Jaws Dose Algorithm: In non-jaws situations, the following algorithm is used:
SAD2 D (d , x, y, z) M S TMR (d ) OAR ( x , y ) n t n n 0 0 SAD Z 2 n
(6)
Where, n refers to the circular collimator which was used, and x0, y0 are x and y Projected onto the isocentric plane. [II]Jaws Dose Algorithm: When jaws are used to shape the beam, Xknife uses this dose algorithm:
D(d , s jaw , snet , x, y, z ) M J S J ( s jaw ) S P ( snet ) TMR(d , snet )
OARn ( x0 y0 ) Px1, x 2 (dis( x, y ), P( z )) Py1, y 2 (dis( x, y ), p( z ))
(7)
2
SAD SAD Z 2
Where j is the Jaw Calibration factor, s is the cone size in millimeters, x,y,z are the coordinates of the calculation point in phantom or patient (0,0,0 is isocenter coordinate), P(dis(x,y),p(z)) Penumbral Fit Equation and SAD is the source to axis distance (100 cm). Most often accelerators are calibrated in a fixed source to surface distance (SSD) even though treatments are performed isocentrically (16). In such situations, inverse square correction is needed for the isocentric treatment. Since a dedicated SRS unit is used only in isocentric mode, there is no need to calibrate the unit in the SSD situation Equation (6,7) can be further modified, since M can be made unity . For quick and manual computation of monitor unit to deliver a given dose, a functional form for the TMR is advocated. TMR (d,s) = A(s) * exp [-µ(s)d]
(8)
Where A(s) and µ(s) are the fitting parameters from the measured data for a given cone, s. where TMR has an approximately exponential falloff with depth. Therefore, Xknife uses measured TMR data for dose calculation for points in the range of measurements and an exponential fit for points outside the measurement range, and then, the equation can be rewrite as: Dose = A(s) x exp [-µ(s) x depth].
(9)
The required MU to deliver a prescribed dose, PD can be calculated if the weight of arc, W, and average depth, dav is known. In SRS/SRT, weight of the beam is defined as the ratio of the arc angle of a beam to the
total arc angles of all the beams. This definition of beam weight is slightly different than usually used in external beam. MU = PD* W / OF * A * exp [-µdav]
(10)
The three dosimetric parameters in Eq. (6) as defined in Eqs. (11 12) were measured. TMR (d, s) =
D(d, s,0) D(d max , s,0)
OAR(x, y, z, s) =
D(x, y, z, s) D(0,0, z, s)
(11)
(12)
In Xknife the reference depth of measurements is 5cm, i.e. z =5 RESULTS AND DISCUSSION Mechanical stability The physical dimensions of the cones were measured with film and verified dosimetrically in x and y axes to be within ±0.4 mm. It ensures that collimator rotation will not perturb the dose distribution. The systematic verification of the mechanical and geometrical alignment of the system was undertaken. The mechanical accuracy of lasers, head frames, and gantry was verified with film and it was found within 0.4 mm using cones and radiopaque ball assembly attached to the MIS in various gantry rotations. Similarly, the couch rotation was verified with the radiopaque ball attached to RLPP and LCMA. The isocenter positions recorded on film with the radiopaque ball and cone were within 0.3 mm in various couch angles as shown in (figure 2). The isocenter wander due to gantry, and couch rotations was ≤ 0.5 mm with a compound accuracy of ≤ 0.9 mm. For a normal patient with uniformly distributed weight the table sag was ≤ 0.5 mm. Such small sag is easily corrected with the help of three microadjustments on the LCMA in three directions. The accuracy of Patients' immobilization was excellent, over twenty patients with 95 setups and 2375 scalp measurements. Ninety two percent of measurements had no displacements from the baseline readings, seven percent had 1 mm displacement from the baseline readings and one percent had 2 mm displacements from the baseline readings. the deviations of the error of readjust LTLF according to RLPP was found 0.94, 0.47, and 0.26 mm for Vertical, anterior posterior (AP), and Lateral position respectively.
Dosimetric analysis The TMR data presented in Table (1) and Figure (3) were measured with the pin-point ionization chamber detector for the different cone sizes of the dedicated SRS/SRT unit. The variation of dmax, with cone size seems erratic, especially small cones. This was due to experimental uncertainty. The dmax can be treated at a constant depth of 1.5 cm within the limits of experimental accuracy. The measured and calculated TMR data presented in Figure(3).For larger cones the TMR data were verified to be within ±0.5%. It was reported that at 10 cm depth, approximately 8% increase in TMR can be observed from the 12.5 mm cone to the 40 mm cone. In general, the larger cones have higher TMR values. For our dedicated system the variation in TMR is limited to < 5% between the 12.5 - and 40.0- mm cones. A maximum increase of 8% in TMR was noted between the smallest and the largest cones as in the table. The cone relative output OF(s) for different cone sizes were shown in Figure (4). The penumbra widths are typically 2.38 mm and cones sizes measured within ±0.2 mm as shown in figure (5). The OAR was computed from the beam profiles, however the beam profiles measured in the x and y directions and found within ±0.2 mm, the film and ion chamber provided nearly identical data as shown in (Figure 6a and 6b) for cone12.5 mm and 40.0 mm. Also that found within the same limits for the all nominated cones. The measured OARs were found to be relatively independent of depth. The same observation was also noted by Lutz et al. Since most cranial tumors are within a range of 5 cm, the OAR at depth 5 cm was entered in the treatment planning system for dose calculation. The OAR values entered in the treatment planning system are the average of the x and y profiles. Dosimetrically when multiple arcs are used, the MU/degree could be significantly low. The system was verified for linearity in dose and dose rate. Most often linear accelerators do not provide dose linearity for < 10 MU (17). The dosimetry in lowest (0.5 MU/degree) and highest (10 MU/degree) rotations was verified to be < 1%. The machine was calibrated isocentrically to deliver 1 cGy/MU for a cone of 40 mm at 98.5 cm source to surface distance eliminating the need for the inverse square correction.
Table 1: Measured TMR of Siemens Primus with various Cones. TPR depth cm 0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50 5.00 5.50 6.00 6.50 7.00 7.50 8.00 8.50 9.00 9.50 10.0 10.50 11.00 11.50 12.00 12.50 13.00 13.50 14.00 14.50 15.00 15.50 16.00 16.50 17.00 17.50 18.00 18.50 19.00
1.25 cm
1.5 cm
1.75 cm
2 cm
2.25 cm
2.5 cm
2.75 cm
3 cm
3.25 cm
3.5 cm
3.75 cm
4 cm
42.81 85.7 95.76 100 96.94 95.75 92.56 92.31 90.47 88.37 85.47 83.8 81.21 80.32 79.08 74.21 74.67 72.73 70.18 69.77 67.82 65.01 63.92 63.17 61.42 60.37 59.42 56.2 56.19 54.01 52.69 51.59 51.26 49.46 49.22 47.63 46.69 45.86 45.69
42.8 85.65 95.89 100 97.14 95.74 92.8 92.38 90.5 88.26 85.37 83.91 81.69 80.27 79.12 75.55 74.63 73.09 71.21 70.35 68.28 66.13 64.84 63.43 62.88 60.8 60.23 57.8 57.77 55.52 54.28 53.69 53.02 51.11 50.49 49.46 47.51 47.36 45.58
83.48 98.44 100 98.76 97.16 95.72 93.25 91.1 89.26 86.38 85.08 82.43 80.78 79.2 75.86 75.41 73.16 71.13 70.45 68.23 65.96 64.55 63.38 61.72 60.46 59.44 56.47 56.2 54.17 52.76 51.57 51.13 49.32 49.08 47.56 46.6 45.86 45.53 43.55
84.4 97.39 100 99.36 96.91 96.06 93.93 91.22 89.78 87.67 86.07 84.24 81.75 79.34 77.96 76.62 74.32 72.99 71.82 69.46 68.42 66.55 64.58 63.33 61.56 60.38 58.75 57.46 56.81 55.24 53.71 52.33 51.12 50.72 49.31 48.2 47.02 45.72 44.86
82.94 97.39 99.88 100 97.75 95.88 93.63 91.43 89.47 88.45 85.7 84.44 82 79.53 78.48 76.76 74.91 73.45 71.56 69.85 68.61 66.7 64.95 63.78 62.18 60.67 58.73 58.5 56.64 55.55 54.13 53.45 52.47 51.06 49.79 48.58 47.38 46.32 45.46
42.56 80.97 96.76 100 98.8 98.08 96.15 93.38 90.97 89.12 88.08 85.72 83.6 81.8 80.08 78.54 76.59 74.94 73.61 71.14 70.62 68.35 66.54 65.53 64.15 61.73 60.82 58.95 58.92 56.69 55.38 54.11 53.27 52.57 51.29 49.99 48.67 47.57 46.69
42.56 75.96 94.85 100 99.385 98.475 96.69 94.32 92.295 90.2 88.875 86.55 84.885 82.85 80.83 78.95 77.31 75.91 74.445 71.54 71.02 68.75 66.94 65.93 64.55 62.13 61.22 59.35 59.32 57.09 55.78 54.51 53.67 52.97 51.69 50.39 49.07 47.97 47.09
70.95 92.94 100 99.97 98.87 97.23 95.26 93.62 91.28 89.67 87.38 86.17 83.9 81.58 79.36 78.03 76.88 75.28 71.94 71.42 69.15 67.34 66.33 64.95 62.53 61.62 59.75 59.72 57.49 56.18 54.91 54.07 53.37 52.09 50.79 49.47 48.37 47.49 46.15
40.52 81.96 96.16 100 99.34 98.52 96.69 95.26 93.92 90.92 88.95 87.37 85.56 83.82 82.83 79.26 78.45 77.23 75.63 72.29 71.77 69.5 67.69 66.68 65.3 62.88 61.97 60.1 60.07 57.84 56.53 55.26 54.42 53.72 52.44 51.14 49.82 48.72 47.84
78.86 94.73 100 99.61 99.04 97.31 95.98 94 91.44 89.36 86.85 85.56 83.27 82.14 79.7 78.75 77.53 75.93 72.59 72.07 69.8 67.99 66.98 65.6 63.18 62.27 60.4 60.37 58.14 56.83 55.56 54.72 54.02 52.74 51.44 50.12 49.02 48.14 46.8
40.92 82.36 96.56 100.4 99.74 98.92 97.09 95.66 94.32 91.32 89.35 87.77 85.96 84.22 83.23 79.66 78.85 77.63 76.03 72.69 72.17 69.9 68.09 67.08 65.7 63.28 62.37 60.5 60.47 58.24 56.93 55.66 54.82 54.12 52.84 51.54 50.22 49.12 48.24
80.7 96.45 100 99.04 97.7 96.7 94.64 93.37 91.32 88.29 87.09 85.48 83.74 82.44 80 79.05 77.83 76.23 72.89 72.37 70.1 68.29 67.28 65.9 63.48 62.57 60.7 60.67 58.44 57.13 55.86 55.02 54.32 53.04 51.74 50.42 49.32 48.44 47.1
CONCLUSION A dedicated unit such as the Primus M6/6ST saves a significant amount of time for retrofitting cone assembly, positioning, aligning, QA, and verification, as well as improving the precision in the dose delivery. The precision in such units could be achieved to be within 0.5 mm with a compounded accuracy of < 1 mm for SRS and < 3 mm for the SRT. Originally the system was verified for the cones in the range of 12.5-40 mm cones. The above QA process ensures that the couch and gantry rotations are in agreement with the isocenter and target. Dosimetric parameters, TMR, OAR, cone factors and dose linearity of primus M6/6ST are presented based on the measurements with ion chambers. The mechanical stability, smaller penumbra, and dosimetric properties of Primus M6/6ST are better compared to the add-on linear accelerators. The system is relatively easy, friendly, reliable, and may be relatively cost effective. REFERENCES (1) L. Leskell, "The stereotactic method and radiosurgery of the brain.", Acta Chir. Scand. 102: 316-319, (1951). (2) Larson, B., K. Liden, B. Sorby. "Irradiation of small structures through intact skull" Acta Radiol. Ther. Phys. Biol. 13:513-534 (1974). (3) Betti, O. O., and V.E. Derechinsky. "Hyperselectective encephalic irradiation with linear accelereator." ActaNeurochir. Suppl. (Wien) 33:385-390 (1984). (4) Colombo, F., A. Benedetti, F. Pozza, R. C. Avanzo, C. Marchetti, G. Chierego, A. Z. enerdo. "External stereotactic irradiation by linear accelerator." Neurosur. 16:154-160 (1985). (5) Hartmann, G. H., W Schlegel, V. Sturm, B. Kober, O. Pastyr, W. J. Lorenz. "Cerebral radiation surgery using moving field irradiation at a linac facility" Int. J. Radiat. Oncol. Biol. Phys. 11:1185-1192 (1985). (6) Lutz, W., Winston, K.R. and Maleki, N. "A system for stereotactic radiosurgery with a linear accelerator." Int. J. Radiat. Oncol. Biol. Phys. 14: 373-381, (1988). (7) Podgorask, E. B., A. Olivier, M. Pla, P. Y. Lefebvre, J. Hazel. "Dynamic stereotactic radiosurgery." Int.J. Radiat. Oncol. Biol. Phys. 14:115-126 (1988). (8) Adler, J.R., and R. S. Cox. "Preliminary Clinical Experience with the Cyberknife; Image Guided Stereotactic Radiosurgery." In Radiosurgery. D. kondziolka (Ed.). (Basel: Karger), pp. 317-326, (1991). (9) Mackie, T. R., T. W. Holmes, P. J. Rechwerdt, J. Yand. "Tomotherapy: Optimized planning and delivery of radiation therapy." Int. J. Imaging Systems and Technol. 6:43-55 (1995). (10) Podgorsak, E.B. Physics for radiosurgery with linear accelerators. Neurosurg. Clin. North Am. 3: 9-34, (1992).
(11) Brenner, D.J., Martel, M.K. and Hall, E.J. Fractionated regimes for stereotactic radiotherapy of recurrent tumors in the brain.Int. J. Radiat. Oncol. Biol. Phys. 21: 819-824, (1991). (12) Van Dyk, J. "The Modern Technology of Radiation Oncology." Medical physics Publishing, Madison, Wisconsin. (2000). (13) Indra J. Das, M.Beverly Downes, Benjamin W. Corn, Walter J. Curran, M. Werner-Wasik, David W. Andrews "Characteristics of a dedicated linear accelerator-based stereotactic radiosurgeryradiotherapy unit." Radiotherapy and Oncology 38: 61-68, (1996). (14) Tsai, J.S., Buck, B.A., Svensson, G.K., Alexander, E., Cheng, C.W., Mannarino, E.G. and LoeBler, J.S. Quality assurance in stereotactic radiosurgery using a standard linear accelerator. Int. J. Radiat. Oncol. Biol. Phys. 21: 737-748, (1991). (15) AAPM Report Number 54. "Stereotactic Radiosurgery" American Institute of Physics, Woodbury, NY, (1995). (16) Bjamgard, B.E., Bar-Deroma, R. and Corrao, A. A survey of methods to calculate monitor settings. Int. J. Radiat. Oncol. Biol. Phys. 28: 749-752, (1994). (17) Das, I.J., Kase, K.R. and Tello, V.M. Dosimetric accuracy at low monitor unit settings. Br. J. Radiol. 64: 808-811, (1991).
Capture Figure (1): Film Test with RLPP. Figure (2): Couch Rotation Film Test. Figure(3):Tissue Maximum Ratio for cone12.5mm and 40.0 mm; these data were taken with Wellhofer dosimetry system. Figure (4): The cone relative output for different field sizes. Figure (5): Differences in actual and measured (50%- isodose) cone diameter and average penumbra (80-20%) dimensions for all cones. Figure (6a): Dose profile for cone 12.5mm.these data were taken with Wellhofer dosimetry system using pinpoint ion chamber. For the film measurement, a Laser Densitometer was used to get OAR needed for the treatment planning system. Figure (6b): Dose profile for cone 40.0mm.these data were taken with Wellhofer dosimetry system using pinpoint ion chamber. For the film measurement, a Laser Densitometer was used to get OAR needed for the treatment planning system.
Pictures
Figure 1
Gantry with RLPP
Figure 2
Cone12.5 mm
Tissue Maximum Ratio (TMR)
120 100 80 Measured TMR
60
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Cone 40.0 mm Tissue Maximum Ratio (TMR)
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Figure 3
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cones relative output cGy/MU 1.00
Normalized output cGy/MU
0.98 0.96 0.94 0.92 0.90 0.88 0.86 0.84 10
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Figure 4 3 Penambra Actual-50% 2.5
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Figure 5
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Figure 6a
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Figure 6b
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